--- title: "Parts that fit" subtitle: "Design and print · Designing to size · Lesson 2" --- # Parts that fit **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** The lid you designed is exactly the size of the box's opening. You print both, and the lid won't go on. Nothing was measured wrongly. What happened? :::teacher-note **In one sentence:** Students see that a same-size peg and hole won't fit once printed, open the hole up to a sliding fit, work out a hole size from the peg, the printer's effect and the clearance wanted, design a press fit and a different peg size, print a tolerance test card, and design a lid peg's hole. **Students should be able to say:** I can explain why same-size parts don't fit when printed. I can work out a hole size for a sliding or press fit. I can calibrate a design to our printer with a test card. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **A 10 mm peg and a 10 mm hole are printed. The peg…** ☐ Fits perfectly ☐ Won't go in ☐ Fits, but only one way round ☐ Slides in loosely My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It won't go in: the hole prints at about 9.8 mm and the peg at about 10.1 mm. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: design the hole :::keep-together ### Work it out 1. $\text{peg drawn } 10 \to \text{printed } 10.1$ — Pegs print about 0.1 mm bigger. 2. $\text{want clearance } 0.3$ — A sliding fit. 3. $\text{hole printed} = 10.1 + 0.3 = 10.4$ — The hole must come out this big. 4. $\text{hole drawn} = 10.4 + 0.2 = 10.6$ — Holes print 0.2 mm smaller, so draw it bigger. Now check it with the numbers. **A 10 mm peg, a sliding fit with 0.3 mm of clearance after printing. What size should the hole be drawn (mm)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 10.6 printed peg 10.1, so the printed hole must be 10.4; drawn 10.4 + 0.2 = 10.6 mm. ::: :::keep-together ### Find the mistake **Sam's lid peg won't go in its hole. Click the line where Sam's reasoning first goes wrong.** ``` 1 The lid's peg is 10 mm across. 2 So the hole in the box should be 10 mm across too. 3 Same size means a perfect fit. 4 Sam prints both and the peg won't go in. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Line 2: the hole needs a planned clearance. Drawn the same, the printed hole is smaller than the printed peg. ::: :::typical ## Vary: change one thing **Press fit.** **A 6 mm peg.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Print a test card Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Fits :::keep-together ### Will it go in? **A 5 mm peg, hole drawn 5.2 mm. Printed clearance, and fit?** ☐ −0.1 mm: won't go in ☐ 0.5 mm: loose ☐ 0.2 mm: sliding fit ☐ 0.1 mm: press fit ::: :::teacher-note **Answer:** −0.1 mm: won't go in Hole prints 5.0, peg 5.1: −0.1 mm. ::: :::keep-together ### Press fit **A 12 mm peg, a press fit with 0.1 mm of clearance after printing. Hole drawn (mm)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 12.4 Peg prints 12.1; hole must print 12.2; draw 12.4. ::: ## Make: a lid peg Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Why won't a 10 mm printed peg go in a 10 mm printed hole?** {{lines-3}} ::: :::teacher-note **Answer:** The printer makes holes smaller and pegs bigger, so the printed hole is smaller than the printed peg. :::